An Analyst Runs an Alkane Standard on a GC-MS Prior

An analyst runs an alkane standard on a GC-MS prior to running a sample from a case. The following data is obtained:
Retention time of the compound in the sample: 7.21 minutes
Retention time of the C7 test alkane: 6.42 minutes
Retention time of the C8 test alkane: 7.58 minutes
Calculate the retention index of the compound in the sample

A) 821
B) 768
C) 792
D) 702
E) none of the above